2 DefinitionThe resolving power of the eye used to assess and quantify the eye’s ability to resolve varying letter sizesVisual acuity is dependent on:The refractive error of the eyeThe health and the integrity of the eyeThe test targets usedThe test conditions

3 MeasurementThe measurement of VA is done to monitor change in vision with disease progression of disease and/or treatment planIt is a way to measure “blur” visionVisual acuity must be measured on every patient at every visit!

5 Weymouth Classification of Visual Acuity MeasurementsMinimum distinguishable (or minimum visible)The ability to see something as being distinguishable from the backgroundMinimum separableThe ability to determine a group of points or lines as separate and distinctMinimum cognizable or legibleThe ability to form sense (e.g., Landolt C) or minimum legible (e.g., letters or numbers)

6 HistoryEarly astronomers found two stars resolvable as separate if the distance between the stars subtend an angle of at least 1 minute of arc

7 Snellen TestIn 1862 Snellen and Giraud-Toulon introduced a system for measuring VAA form of minimum legible visual acuity taskSnellen acuity is based on a minimum angle of resolution of 1 minute of arcVisual acuity target designs: optotypesThe Snellen optotype has overall dimensions that are 5 times the detail

9 Snellen Notation Snellen fractionTest distance divided by the distance at which optotype would subtend 5 minutes of arc at the retinaThe denominator of the Snellen fraction is termed the foot-letter sizeThe letter’s overall size, both angularly and linearlyThe average emmetrope has a VA of 20/20 or betterThis is based on a test distance of 20ftThe classic Snellen fraction is the reciprocal of the minimum angle of resolution (MAR)

10 Snellen Acuity and Minimum Angle of ResolutionFractionDecimalMinutes of Arc20/201.0120/250.801.2520/300.671.520/400.50220/500.402.520/1000.20520/2000.1010

12 Snellen Test Snellen equivalent ExampleIf the test distance is not 20ft, the measured Snellen fraction may be used to predict what the patient’s VA would be for a 20ft distanceExamplePatient can resolve a 30ft letter at a test distance of 15ft. The Snellen acuity is 15/30.15/30 =20/xx = 40The Snellen equivalent is 20/40

13 Calculating a Visual Acuity Test Object’s Foot-Letter DesignationExample of a letter that is 2cm tall

18 Types of Charts Hand-held visual acuity cardsPrimarily for children in vision screeningsSimplest form is the letter E in different sizes printed onto hand-held cardsFree space testing

19 Types of Charts Chart projectors Projects test objects onto a screenStandard chart projector can be calibrated for varying testing distances between 10ft and 20ftOptically “fold” the testing distance using mirrors to create appropriate testing distance (20ft is the standard)

20 Calibration of Chart ProjectorsMeasure the testing distanceCalculate how large an optotype would have to be on the screen to represent 20/200 for that testing distanceAdjust the lenses in the chart projector until the letter size from the previous step is achieved

22 Calibration of Chart ProjectorA 20 foot-letter is 8.87mm. May use direct proportions to calculate the letter size that would be needed to represent 20/20 for any possible testing distance.Example8.87mm/20ft = x/17ftx = 7.54mm

23 RecordingRecord the VA line for which more than 2 letters (or less than half the number) are read incorrectly

24 RecordingExamplesTwo out of five letters on the 20/20 line are read incorrectly20/20 -2All letters on the 20/20 line are read correctly, and one letter on the next line down is read correctly20/20 +1One of the letters on the 20/20 line is read incorrectly, and two letters on the 20/15 line are read correctly20/20 -1/+2

25 Recording Points to considerAsk the patient to read a line over again or show the letters to the patient one line at a timeIsolate lines on the chart to avoid distracting influencesMay consider showing the patient a line of the same VA on a different slide, if the projector has more than one slide

26 Expected FindingsA visual acuity of 20/20 or better is considered normalThe difference between the two eyes should be no greater than one line

27 Pinhole Visual Acuity PurposeTo determine if a decrease in vision is correctable by lensesPinhole visual acuities are taken when the patient’s entering VA is worse than 20/30 at distance and near through the habitual correction

28 Expected FindingsIf the cause of the patient’s decreased visual acuity is due to an uncorrected refractive error, the VA is expected to improve through the pinholeIf the cause of decreased visual acuity is not optically based, no improvement, and possibly a decrease, will occur through the pinhole

30 Near Visual Acuity Testing the VA at close range (usually 40cm)Near visual acuities are taken through the habitual correctionThe purpose is to detect people with near vision difficulties (e.g., uncorrected high hyperopia, accommodative dysfunction)In patients over 40 years old, the reduced near visual acuity is one of the symptoms of presbyopia